1. Field of the Invention
The present invention relates to a method of depositing a silicon thin film used in, for example, a thin film solar cell and to a silicon thin film solar cell.
2. Description of the Related Art
A thin film solar cell module is constructed such that string-like solar cells each consisting of a transparent electrode layer, a photovoltaic semiconductor layer, and a back electrode layer, which are stacked one upon the other on a transparent substrate, are connected in series. The photovoltaic semiconductor layer formed of amorphous silicon is low in cost, but is defective in that the photovoltaic efficiency is low. In order to improve the photovoltaic efficiency, it is advantageous to use a hybrid type photovoltaic semiconductor layer in which pin-type amorphous silicon and pin-type polycrystalline silicon (polysilicon) layer are stacked one upon the other or a polysilicon type photovoltaic semiconductor layer using pin-type polysilicon alone. Also, a substrate having a large area has come to be used for improving the manufacturing efficiency of the thin film solar cell module.
In order to deposit a photovoltaic layer on a transparent electrode layer formed on a transparent substrate having a large area, it is efficient to use vertical-type in-line plasma CVD apparatus. The method of depositing a photovoltaic semiconductor layer by using vertical-type in-line plasma CVD apparatus will now be described with reference to FIGS. 1A and 1B. As shown in FIG. 1A, a frame-like substrate holder 1 is constructed to have a recess, slightly larger than a substrate 2, on the back surface. The substrate holder 1 is first placed horizontally, and the substrate 2 is fitted into the recess of the substrate holder 1 from the backside under the state that the transparent conductive film is positioned on the front side. As shown in FIG. 1B, a back plate 3 is put on the back surface of the substrate holder 1, and pins are slid between fixing tools 1a of the substrate holder 1 and fixing tools 3a of the back plate 3 so as to hold the substrate 2. The substrate holder 1 holding the substrate 2 under the particular state is held upright and moved within the vertical-type in-line plasma CVD apparatus to the position of an electrode 4. Under this condition, a photovoltaic semiconductor layer is deposited by plasma CVD. Incidentally, a conductive material such as SUS is used for the substrate holder 1 in view of the mechanical strength required for holding the substrate having a large area.
FIG. 2 shows in a magnified fashion the contact portion between the substrate holder 1 and the substrate in the process of depositing a photovoltaic semiconductor layer by the conventional method. As shown in FIG. 2, a transparent conductive film 22 is formed on a transparent substrate 21, and the peripheral region of the transparent conductive film 22 is in contact with the inner edge portion of the substrate holder 1.
No problem was generated in the case of depositing an amorphous silicon film by plasma CVD under the state shown in FIG. 2. However, in the case of depositing a polysilicon film, abnormal distribution or defects have been generated in the thin film. In the worst case, it has been found that the substrate is cracked. It has been clarified that the difficulty is caused as follows.
Amorphous silicon has a relatively high absorption coefficient and, thus, the thickness of the amorphous silicon film can be decreased. In the case of a polysilicon film, however, it is necessary to increase the thickness of the film because polysilicon has a low absorption coefficient. In order to improve the productive efficiency by shortening the time required for depositing the polysilicon layer, it is necessary to supply high power to the substrate so as to increase the film deposition rate. To be more specific, for depositing a polysilicon layer, the power density on the substrate is set at a high level not lower than 100 mW/cm2. The power density noted above is at least 4 to 6 times as high as the power density for depositing an amorphous silicon layer. If plasma CVD is performed under a power density not lower than 100 mW/cm2 under the state that the transparent conductive film 22 formed on the surface of the substrate 21 is brought into contact with the substrate holder 1 as shown in FIG. 2, problems are generated such as blackish discoloring of the transparent conductive film, defects such as flaws and scrapes, and a substrate crack. These defects are rendered prominent with increase in the supplied power. It is considered reasonable to understand that a charge is accumulated in the transparent conductive film 22 in performing the plasma CVD so as to bring about abnormal discharge (a spark) between the tip of the substrate holder 1 and the transparent conductive film 22, leading to the defects referred to above.
If the substrate holder 1 could be brought into a tight contact with the transparent conductive film 22, it would be theoretically possible to release the charge accumulated on the surface of the transparent conductive film 22 through the substrate holder 1 so as to overcome the difficulty noted above. However, it is practically impossible to bring the substrate holder 1 into a tight contact with the transparent conductive film 22 because of, for example, the warp of the substrate 21.
An object of the present invention is to provide a method of depositing a silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus, which permits improving uniformity of the silicon thin film and also permits preventing a substrate crack so as to realize stable production.